Manual of basic techniques for a health laboratory - libdoc.who.int

280 Manual of basic techniques for a health laboratoryBefore the introduction of SI units, the erythrocyte volume fraction was calledeither the “haematocrit” or the “packed cell volume”, and it was reported as apercentage rather than a decimal fraction. In the traditional system, the “packedcell volume” in the example given would be 45%. Note that, in using SI units, thenumerical value does not change, but becomes 0.45 instead of 45%.9.4.1 Micro-scale methodPrincipleThe blood (mixed with anticoagulant) is placed in a long capillary tube and centrifugedin a microhaematocrit centrifuge. The level reached by the column of erythrocytesis read with a scale reader. This method is preferable to that using a macroscale: it is quicker, and blood from the finger can be used.Materials and reagents (Fig. 9.27)● Microhaematocrit centrifuge● Scale reader (usually provided with the centrifuge)● Capillary tubes, 75mm long with a 1.5-mm bore, containing driedheparin (if capillary blood is used; if venous blood mixed with EDTAdipotassium salt, 10% solution (reagent no. 22) is used, “heparinized”tubes are not required)● Long fine capillary Pasteur pipettes (long enough to reach the bottomof the tubes) with rubber teat● Filter-paper● Soft wax or plastic modelling clay (or a Bunsen burner or spiritlamp)● Sterile blood lancet● 70% Ethanol.Fig. 9.27 Materials for estimating the If no scale reader is available, you can make one yourself using grapherythrocyte volume fractionusing the micro scalepaper, 15–20cm wide, ruled in millimetres. On the left-hand verticaledge, starting at the bottom, make a series of 10 marks at intervals of4mm. On the right-hand vertical edge, in the same manner, make 10 marks atintervals of 6mm. Using a ruler, draw 10 sloping lines connecting each mark on theleft margin to the corresponding mark on the right margin. In the left margin,opposite the bottom horizontal line of the graph paper, write “0”. Continue up theleft margin, marking each sloping line you have drawn as follows: 0.1, 0.2, 0.3, etc.;the top sloping line will be marked 1.0. In the right margin, write the same numbersopposite the other ends of the sloping lines. Now, again using a ruler, draw a secondseries of sloping lines, but make them much lighter than the first set of lines. Eachlight line should be drawn exactly in the middle of the space between each pair ofheavy lines. Finally, following the printed lines of the graph paper, draw a series ofheavy vertical lines at intervals of about 3cm. Your scale should look like the one inFig. 9.28. Instead of making your own scale, you could use the one printed here forreading erythrocyte volume fractions. (Cover it with a sheet of plastic.)MethodCollection of specimensCapillary blood specimens1. Using a blood lancet, draw blood by pricking either:— the third or fourth finger (Fig. 9.29)— the lobe of the ear— the heel (infants)after sterilizing the chosen area with ethanol.